1. Field of the Invention
The present invention relates to improvements of a radio-frequency tag communication device capable of information writing and reading on and from radio-frequency tags by radio communication, a radio-frequency tag communication system including the radio-frequency tag communication device, and a radio-frequency tag detecting system.
2. Description of the Related Art
There is known an RFID system (radio-frequency identification system) wherein a radio-frequency tag communication device (interrogator) is arranged to effect non-contact information reading from small-sized radio-frequency tags (transponders) in which desired information is stored. In this RFID system, the radio-frequency tag communication device is capable of reading the stored information from the radio-frequency tags by radio communication, even where the radio-frequency tags are soiled or located at invisible places. For this reason, the RFID system is expected to be used in various fields such as commodity management and inspection.
The radio-frequency tag communication device usually has a maximum distance of communication of about several meters. Therefore, the use of a single interrogator as the radio-frequency tag communication device does not permit adequate communication with the radio-frequency tags located in a comparatively large space. In view of this drawback, there have been proposed techniques for enlarging the area of communication of the radio-frequency tag communication device. Patent Document 1 discloses an interrogator system as an example of such techniques. According to this technique, the radio-frequency tag communication device consisting of a plurality of interrogators and a control terminal to control the plurality of interrogators is arranged such that the areas of communications of antennas provided in the respective interrogators overlap each other, to permit radio communication between the plurality of interrogators and the radio-frequency tags, using the same signal, so that the overall area of communication of the radio-frequency tag communication device is enlarged.
There is also proposed a technique that uses a plurality of radio-frequency tag communication devices which cooperate with each other to cover a desired area of communication with the radio-frequency tags. Patent Document 2 discloses a navigation system as an example of this technique. This technique uses a plurality of fixed terminals (stationary radio-frequency tag communication devices) located in a desired chamber or any other desired space, and a portable terminal (portable radio-frequency tag communication device) which is carried by the users, to permit adequate detection of or guiding to a desired destination within a relatively narrow area.
Patent Document 1: JP-2003-283367 A
Patent Document 2: JP-2001-116583 A
However, the former conventional technique described above requires the plurality of interrogators to be located comparatively close to each other so that the areas of communications of the antennas provided in the respective interrogators overlap each other, so that the required number of the interrogators tends to be undesirably increased. Further, so-called “passive tags” not provided with an internal power source are generally unable to effect communication by concurrent reception of signals from the plurality of interrogators, and the passive tags located within mutually overlapping regions of the areas of communication are not able to effect adequate communication with the interrogators. Thus, there has been a need of developing a radio-frequency tag communication system which has a sufficiently large area of communication with a minimum required number of the interrogators. On the other hand, the latter conventional technique described above has a risk of an interference between waves transmitted from the plurality of fixed terminals, which interference has an adverse influence on the information reading from the radio-frequency tags. Thus, there has been a need of developing a radio-frequency tag detecting system capable of accurately detecting the radio-frequency tags within the desired area of communication while effectively preventing the interference of the transmitted waves.